Can teenagers no there gay or bi at such a young age?

I remember spotting a boy at school who was very feminine in his movements, looks and attitude. I didn't know anything about homosexuality then and didn't really care about him much as he moved in different circles. He took a lot of bullying though. I'm not too sure if he even knew why he was different at that age, about 12, but he certainly knew his life was miserable because of it. No idea what happened to him later.